1. Field of the Invention
The invention relates to a method of regenerative heat transfer from one flow to another based on the use of circulating fluidized bed reactors. The method can be employed to solve problems associated with conventional fixed-element, fixed-bed and fluidized-bed heat regenerators.
2. Description of the Related Art
Heat transfer from one flow to another is one of the basic unit processes in process and energy technology. Currently, heat transfer is implemented chiefly using two heat exchanger types called recuperative or regenerative according to their operating principle.
In recuperative heat exchangers, heat transfer takes place via a nonpermeable wall which isolates the flows from each other. In the basic type of a recuperative heat exchanger, thermal energy is conducted directly through the wall from one flow to another. A specific subgroup of recuperative heat exchangers is formed by the so-called intermediate circulation recuperators in which a heat transfer medium is recirculated between two recuperative heat exchangers. Such heat exchangers are employed in, e.g., nuclear power plants in which it is necessary to assure that the high-activity flow cannot mix with the secondary circulation in accident situations. Another exemplifying group of intermediate-circulation recuperators is formed by fluidized-bed boilers equipped with superheaters placed external to the combustion chamber; in these boilers the sand heated in the combustion chamber is cooled in a separate fluidized-bed super-heater. The chief limitations of recuperative heat exchangers are related to the erosion, corrosion and temperature of the heat exchanger vessel wall materials. Today, no practical wall materials are available for conditions exhibiting high mechanical or chemical stresses.
The highest allowable temperature in recuperators is often limited by the strength properties of the wall material. Moreover, recuperators are expensive and restricted in their control possibilities. Good controllability can, however, be achieved in intermediate-circulation recuperators.
In regenerative heat exchangers, thermal energy is transferred by way of allowing the heated heat-transferring medium to release energy into a colder flow and then reheating the cooled heat-transferring medium in a hotter flow. Regenerative heat exchangers are further divided into cyclically and continuously operating types on the basis of their operating principle.
In cyclically operating regenerators the hotter and the cooler flow are cyclically routed via a single solid structure which thus alternatingly stores and releases thermal energy. The batch-heated rock stove of a sauna is without doubt the oldest application of the cyclically operated regenerator.
In continuously operating regenerators the heat-storing medium is continually recirculated from one flow to another. The best-known type of continuously operating regenerator is the Ljungstrbm regenerator in which a rotating, cylindrically shaped heat exchanger disc transfers thermal energy from one material flow to another. This regenerator type has been modified for different applications such as, for example, the air-conditioning regenerator which additionally provides moisture transfer on surfaces coated with lithium chloride paste.
Besides the regenerator types of the above-described kinds with a fixed-shape, contiguous heat-transferring element, regenerators based on granular heat transfer media are known in the art.
Several different types of regenerators are known having the granular heat transfer medium in the fixed-bed state and the heat transfer medium is then mechanically recirculated between the layers of the bed.
German patent DE 3,225,838/A/employs a granulated heat transfer medium (e.g., porcelain pellets) for heat transfer between the gas flows. The granular bed material is fluidized, whereby the pellets remain clean and clogging of the heat exchanger is avoided. U.S. Pat. No. 4,307,773 discloses another type of process and apparatus in which a regenerator system based on bubbling fluidized bed layers is employed for heat recovery from contaminating gases. Besides the above-described patents, different types of regenerators are known based on alternate heating/cooling of granular material in separate, parallel, bubbling fluidized bed layers. UK patent 2,118,702 A discloses a regenerator based on downward dribbling fixed bed layers.
A central issue of regenerators based on a fixed heat transfer element and fixed layered zones of granular material is how to keep them clean. Also the need to prevent flows from mixing with each other causes sealing problems in these regenerators. Furthermore, the temperature differentials formed the heat transfer material impose mechanical stresses which limit the life of the heat transfer element or material. A drawback of the layered fixed bed regenerator is the channelling of flows in the fixed bed layers. Moreover, the fixed bed layers obviously develop inevitable temperature gradients in the direction of the flow and the temperature of a layer is difficult to control.
In fluidized bed reactors the flow velocities must be adapted according to the physical properties of the heat transfer material employed, and the control range of the regenerator is limited between the minimum fluidization velocity and the pneumatic transportation velocity. In practice this means that the heat transfer medium of the regenerator must have a coarse granular size, or alternatively, the flow velocities employed must be kept low. Furthermore, the recirculation of the heat transfer medium between the fluidized bed layers in a manner avoiding excessive mixing of the layers is problematic. This problem is accentuated at high pressure differentials between the heat-transferring flows. Herein, it is generally necessary to use mechanical valves whose wear and temperature limitations eliminate an essential portion of the benefits of this regenerator type. Prior-art fluidized-bed and fixed-bed regenerators require the use of a mechanical or pneumatic conveyor for recycling the heat transfer medium from the lower unit to the upper unit. In terms of equipment and process technology, such conveyors are almost impossible to implement. In fact the recycling system of the granular heat transfer medium is a characterizing property of the present regenerator invention.